Metadata comes in three main types, as definted by the Library of Congress. There are administrative, descriptive and structural metadata according to their system. It often becomes necessary to add several more varieties, or to expand the scope of the already mentioned categories when the digital library becomes more customized towards its contents. For example, librarians (the traditional type) use special types of metadata to manage the distribution of their materials, these would not properly fall into the three categories outlined by the Library of Congress and would better fall into a category of library organization metadata.
Three of the most common metadata formats are Dublin Core, METS and MPEG-21.
Dublin Core is the original metadta standard and as such it is somewhat outdated and derided by professionals. It contains 15 metadata elements which are very constricting and difficult to meld to disparate collections.
METS is the Library of Congress' special metadata format. It is designed to be malleable so that it can be adapted to any number of projects. In this vein the METS format contains many subformats such as MODS and as these new metadata plug-ins are developed there will be a marked increase in the amount of METS used, and probably a corresponding decrease in the use of Dublin Core.
MPEG-21 was commercially developed by the Moving Pictures Expert Group to be applied to multimedia specifically. As it was developed for specific formats, it is likely that it will be used by anyone attempting to build a digital library out of those formats specifically.